Dosing pump and method for enhancing dosing precision

ABSTRACT

The invention relates to a method for enhancing the dosing precision of a dosing pump which is driven by an asynchronous motor via an eccentric gear. The supply of the motor is cut off approximately half a cycle time before a desired dosing pause period or stoppage. The motor freewheels afterwards for part of the cycle time, before a direct voltage is applied to it for approximately a quarter cycle time, such that an eddy current is induced in the rotor, which produces an antipolarized magnetic field which brakes the rotor.

BACKGROUND

1. Field of the Invention

This invention relates generally to methods and apparatus for dosingliquids, and more particularly to a method and apparatus for increasingthe dosing precision of a dosing pump.

2. Discussion of Related Art

Dosing pumps are used for the precision dosing of different liquids.Typically a dosing pump is actuated by external pulses, by a standardsignal or by an internal clock. The external pulses can also begenerated, for example, by a liquid counter and optionally multiplied ordivided. Through use of an eccentric gear, the diaphragm or the pistonof the pump is supposed to complete a stroke, the maximum strokefrequency generally amounting to about 125 strokes/minute.

Asynchronous motors are preferably used for dosing for metering pumpswith a motor output of about 40 watts or higher. Through their moment ofinertia, asynchronous motors are capable of slowing down without brakingwhen switched off. This results in overdosing. With motor outputs ofabout 40 to 60 watts, the slowing down responsible for this overdosingcan be reduced, but not avoided, by stiffening the mounting or byfitting a pressure-retaining valve. A particular problem can arise wherethe diaphragm is driven through an eccentric gear. If the motor comes toa stop outside its dead center, the starting torque for the next cyclecan exceed the permitted value so that the motor does not start.

Mechanical brakes are unsuitable for stroke frequencies of more than 3-strokes/minutes. Hitherto, electrical brakes have only been practicablefor motors with an output of more than 1 kilowatt and for braking timesof more than 1 second. In the case of electromagnetic braking, theelectromagnetic forces are in any case only used to drive mechanicalbrakes. With eddy-current brakes, additional external parts arenecessary which is expensive and makes such brakes impossible to use forsmall motors.

SUMMARY OF THE INVENTION

If exact dosing is to be achieved, controlled braking of the pump motoris essential. Accordingly, the problem addressed by the presentinvention is to develop a method for braking the motor of a dosing pumpin such a way that the diaphragm or the piston of a dosing pump comes toa stop in the exact dead center position after the suction stroke. Thedead center position after the suction stroke is also known as the reardead center. Another problem addressed by the invention is to develop adosing pump to carry out this method which would not require additionalparts on the motor. Braking times of well below 1 second are desired,and be suitable for a range of motor sizes.

The problems stated have been solved by one embodiment of the inventionin which the motor supply voltage to a motor of a dosing pump isswitched off about half a cycle time before a required dosage stoppage,the motor then running freely for part of the cycle time before a d.c.voltage is applied to the motor for about one quarter of the cycle timeso that an eddy current is induced in the rotor which generates acounter-magnetic field that brakes the rotor to a standstill. Anotherembodiment of the invention a dosing pump includes a motor driven by amotor supply voltage adapted to be switched off about half the cycletime before a desired stoppage, the self induction voltage in thefree-running motor then falls to a value which does not affect a brakingcircuit, after which a d.c. voltage is applied to the motor for aboutone quarter of the cycle time so that an eddy current is induced in therotor and generates a counter-magnetic field which brakes the rotor to astandstill.

The advantage of the method according to the invention is that the motoritself is the brake and no additional parts are needed. A particularadvantage is that the d.c. voltage can be applied by simple means. Thed.c. voltage required for braking may be taken from a battery or anaccumulator. However, it may also be provided directly or bytransformation from the operating voltage or, with particular advantage,by rectification and storage in a capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are described in detail below withreference to the drawings, in which the items are identified by the samereference designation, wherein:

FIG. 1 is a block schematic diagram of one embodiment at the invention;and

FIG. 2 is a composite flowchart and timing chart with motor and brakingcurrent waveforms, for an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, in one embodiment of the invention, an ACoperating voltage is applied via a voltage bus 2 to motor current driver4 for providing power to an asynchronous motor 6. The motor 6 drives aneccentric gear 14 to operate a dosing pump 16. A control circuit 8 isprovided for controlling the motor current driver 4, and a brakingcurrent driver 10. The AC voltage from bus 2 is applied to an AC/DCconverter or rectifier 12, for providing a DC voltage to braking currentdriver 10. Please now refer to both FIG. 1 and FIG. 2. Operation of theinvention relative to an embodiment for one method of the invention willnow be described.

A time framework is drawn up by way of example in the following for adosing pump with a cycle time of 500 ms which substantially correspondsto a stroke frequency of about 125 strokes/minute. In the stroke inwhich braking is to take place, the operating or supply voltage is stillin place from the beginning of the cycle in the rear dead center for theduration of half a stroke of 250 ms. After passing through the frontdead center, the supply voltage is switched off and, after about 100 ms,the motor is connected to a d.c. voltage for about 125 ms. After a timeinterval of about 25 ms, the cycle begins again. The application of ad.c. voltage generates an eddy current which dissipates the energy ofthe moment of inertia and stops the pump after completing the stroke inthe rear dead center position until it is actuated again. Accordingly,the pump has completed exactly one stroke and can start up with nocounter-moment in the next cycle. This sequence leads to a considerableincrease in dosing precision and in the operational reliability of thepump.

In one embodiment of the invention, the braking current driver 10 isonly activated when the actuation interval or time for the dosing pump16 to complete a full stroke at least corresponds to the drive cycletime thereof. As indicated in the above example, for a drive cycle timeof 500 ms, the dosing pump completes each stroke within 500 ms.

Although various embodiments of the invention have been shown anddescribed, they are not meant to be limiting. Those of skill in the artmay recognize certain modifications to theses embodiments, whichmodifications are meant to be covered by the spirit and scope of theappended claims.

What is claimed is:
 1. A method for increasing the dosing precision of adosing pump which is driven by an asynchronous motor via an eccentricgear, said method comprising the steps of:switching on a supply voltageto said motor for driving said dosing pump for initiating an actuationinterval; switching off said supply voltage to said motor about half acycle time before it is required to stop said motor to terminate theactuation interval for dosing by said dosing pump; letting said motorrun freely in an unpowered mode for about one quarter of the cycle timeafter said switching off step; and applying a DC voltage to said motorfor about the next one quarter of the cycle time for inducing an eddycurrent in a rotor of said motor to generate a counter-magnetic fieldfor breaking said rotor to a standstill.
 2. A method as claimed in claim1, characterized in that, wherein between the step of switching off ofthe supply voltage and application of a DC voltage, the motor runsfreely until, after about 20% of the cycle time, the self-inductionvoltage has fallen to a value which does not affect the braking.
 3. Amethod as claimed in claim 1 further including the step of switching offthe supply voltage when the drive is in a dead center position after acompression stroke of the dosing pump.
 4. A method as claimed in claim1, wherein the motor is at a standstill for about 5% of the cycle timebefore reactuation in a dead center position after a suction stroke. 5.A method as claimed in claim 1, wherein the motor brake is onlyactivated when the duration of the actuation intervals at leastcorresponds to the drive cycle time of the dosing pump.
 6. A dosing pumpdriven by an asynchronous motor through an eccentric gear, wherein saiddosing pump is operated by the method claimed in claim
 1. 7. A method asclaimed in claim 2, further including the step of switching off thesupply voltage when the drive is in a dead center position after acompression stroke of the dosing pump.
 8. A method as claimed in claim2, wherein the motor is at a standstill for about 5% of the cycle timebefore reactuation in a dead center position after the suction stroke.9. A method as claimed in claim 3, wherein the motor is at a standstillfor about 5% of the cycle time before reactuation in the dead centerposition after the suction stroke.
 10. A method as claimed in claim 2,wherein the motor brake is only activated when the duration of theactuation intervals at least corresponds to the drive cycle time of thedosing pump.
 11. A method as claimed in claim 3, wherein the motor brakeis only activated when the duration of the actuation intervals at leastcorresponds to the drive cycle time of the dosing pump.
 12. A method asclaimed in claim 4, wherein the motor brake is only activated when theduration of the actuation intervals at least corresponds to the drivecycle time of the dosing pump.
 13. A dosing pump driven by anasynchronous motor through an eccentric gear, wherein said dosing pumpis operated by the method claimed in claim
 2. 14. A dosing pump drivenby an asynchronous motor through an eccentric gear, wherein said dosingpump is operated by the method claimed in claim
 3. 15. A dosing pumpdriven by an asynchronous motor through an eccentric gear, wherein saiddosing pump is operated by the method claimed in claim
 4. 16. A dosingpump driven by an asynchronous motor through an eccentric gear, whereinsaid dosing pump is operated by the method claimed in claim 5.